A battery back-up power supply is a desirable feature to incorporate into many electronic systems or subsystems. It especially is desirable in connection with memory to prevent the loss of data stored in the memory upon failure (or disconnection) of the primary power supply of the system. In this regard, recent advances in cost-effectiveness and availability of CMOS RAM integrated circuits which require low-standby power are responsible, at least in part, for increased industry interest in adding non-volatility to read/write random access memory in electronic systems.
In the past, various electrical circuits have been devised for automatically connecting a back-up power supply to memory when the normal power supply fails. Similarly, various back-up power supply circuits have been proposed wherein it is an objective to provide the capability to remove a memory module from its host electronic system and transport the module elsewhere without loss of data.
Efficient space utilization and low cost are generally desirable objectives for the packaging of a back-up power supply. For efficient space utilization, it has been proposed that a back-up power supply be packaged in a module having pins extending from the bottom of the module for engaging the sockets of a dual-in-line printed circuit board connector and the module having dual-in-line socket contacts on the top of the module for engaging the pins of a dual-in-line packaged integrated circuit memory. Such a vertically-stacked arrangement is advantageous inasmuch as printed circuit board area required for a dual-in-line circuit board connector is not increased by the addition of such a module. In this particular arrangement, each socket on the top of the back-up power supply module is electrically connected to a corresponding pin on the bottom of the module. Accordingly, the back-up power supply circuitry is connected electrically in parallel with the normal power supply of the system. Such an arrangement may be suitable in applications where the objective is the capability of removing the memory from the host system without loss of data. In that type of application, the back-up power supply module and the integrated circuit memory are removed as a unit from a connector mounted to a printed board of the host system. This arrangement, however, is not suitable for providing power to the associated integrated circuit memory if the normal power supply of the system were to fail by becoming short-circuited. Moreover, this arrangement and many other of the prior art generally provide relatively limited functional features by comparison to the present invention; such prior art circuitry typically only provides means for providing back-up power plus means for inhibiting either the chip enable or write enable input to an integrated circuit memory during power failure or disconnection.
There are other proposals for providing back-up power to an integrated circuit without increase in printed circuit board area beyond that of a conventional connector. One such proposal is depicted in Electronic Design, July 12, 1984, page 183. The commercial version of this approach includes two lithium batteries secured in recesses within a structure which is connected in a piggy-back fashion onto the top of an integrated circuit package containing a non-standard CMOS RAM chip. The integrated circuit package which contains the CMOS RAM chip includes a customized lead frame for interconnection of the batteries to the chip. The CMOS RAM chip may be regarded as non-standard inasmuch as battery back-up circuitry is included in the circuitry therein. Consequently, although this version conforms to a popular standard pin assignment and thus can be interchanged with standard CMOS RAMS available from other manufacturers, replacement of this commercial version with a pin-compatible standard CMOS RAM would result in loss of the non-volatility function.
In accordance with the foregoing, a need exists for a space-saving back-up power supply usable in conjunction with interchangeable standard socket-pluggable integrated circuits and having improved functional features.